There's something wrong with a game design if it is both single player and has "pay to unlock" content. I may consider this in a Freemium game, but not one where you pay once for the game and then again to unlock stuff that came with it. That's like buying a car and then having to buy separate keys for the glovebox. You could just never use the glovebox, but if I knew I wasn't going to get the glovebox on this model I may opt for a different car altogether. (This is not the same reasoning as XM radio or OnStar being installed, those are third-party services.)
***Hello I'm a 30 hour RPG/FPS game that cost 70$ to buy, and I'm going to not let you finish the boss without buying the EZ-MODE switch for 5$. Once you buy this EZ-MODE switch, you'll find it useless to complete the FINAL boss, which you won't be able to win without buying the "DEUS EX MACHINA" button for 15$. Or you could spend the additional 90 hours to level grind your character and save 20$.*** I'm not buying that game in the first place if I know that's in it.
A fun game, players will buy optional content extensions that make logical sense to have. Hello, Warcraft II: Beyond the Dark Portal, circa 1996. A not-fun game however, will not have enough players buy the expansion, and thus the multiplayer possibilities are significantly reduced. Hence there is one problem introduced by expansion packs, is that those who buy the expansion packs can't play with those who haven't. We've been hearing this word a lot "Fragmentation"
The ideal way to solve the need for expansions/episodic gaming without doing EA-insane things (look at the Sims expansion packs) is to actually make the expansions the pay-to-unlock, but still update the game core to prevent fragmentation. So let's say an expansion pack adds characters, weapons, maps and missions. The map/mission is what gives the expansion pack the additional play time, while the characters/weapons do not. If you play with a friend who has bought the expansion pack, that should enable the friend to use the expansion set features with you, who haven't bought the expansion pack, thus providing incentive for you to buy it too.
On the flip side, every time I see the "crate of smurfberries" style of exchanging money for time, I run away. This is just cheating in the guise of "we intentionally make the game painfully hard/time-consuming", there is a reason why freemium game companies have been on a long-term stock slide (Look up Zynga and Nexon), players are just not willing to play their "new" games that have even worse game mechanics that are nothing but pay-to-win.
